Light emitting diodes and laser diodes are well known solid state lighting elements capable of generating light upon application of a sufficient current. Light emitting diodes and laser diodes may be generally referred to as light emitting devices (“LEDs”). Light emitting devices generally include a p-n junction formed in an epitaxial layer grown on a substrate such as sapphire, silicon, silicon carbide, gallium arsenide and the like. The wavelength distribution of the light generated by the LED generally depends on the material from which the p-n junction is fabricated and the structure of the thin epitaxial layers that make up the active region of the device.
Typically, an LED chip includes a substrate, an n-type epitaxial region formed on the substrate and a p-type epitaxial region formed on the n-type epitaxial region (or vice-versa). The substrate may be removed and/or replaced by another substrate in some instances. In order to facilitate the application of a current to the device, an anode contact may be formed on a p-type region of the device (typically, an exposed p-type epitaxial layer) and a cathode contact may be formed on an n-type region of the device (such as the substrate or an exposed n-type epitaxial layer). When a potential is applied to the ohmic contacts, electrons may be injected into an active region from the n-type layer and holes may be injected into the active region from the p-type layer. The radiative recombination of electrons and holes within the active region generates light. Some LED chips include an active region with multiple light emitting regions or active layers (also known as multi-quantum-well structures) between or near the junction of the n-type and p-type layers.
LEDs may be used in lighting/general illumination applications, for example, as a replacement for conventional incandescent and/or fluorescent lighting. As such, it is often desirable to provide a lighting source that generates white light having a relatively high color rendering index (CRI), so that objects illuminated by the lighting may appear more natural. The color rendering index of a light source is an objective measure of the ability of the light generated by the source to accurately illuminate a broad range of colors. In particular, CRI is a relative measurement of how the color rendering properties of an illumination system compare to those of a black-body radiator. A CRI of 100 indicates that the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the black-body radiator. The color rendering index ranges from essentially zero for monochromatic sources to nearly 100 for incandescent sources. For example, daylight has the highest CRI (of 100), with incandescent bulbs being relatively close (about 95), and fluorescent lighting being less accurate (70-85).
In addition, the chromaticity of a particular light source may be referred to as the “color point” of the source. The color point may be defined with reference to a set of tristimulus values (X, Y, Z) and/or color coordinates (CCx, CCy) on a chromaticity diagram. For a white light source, the chromaticity may be referred to as the “white point” of the source. The white point of a white light source may fall along a locus of chromaticity points corresponding to the color of light emitted by a black-body radiator (also referred to herein as a “black body locus”) heated to a given temperature. The black-body locus is also referred to as the “Planckian” locus because the chromaticity coordinates (i.e., color points) that lie along the black-body locus obey Planck's equation: E(λ)=A λ−5/(eB/T−1), where E is the emission intensity, λ is the emission wavelength, T is the color temperature of the black-body and A and B are constants. Accordingly, a white point may be identified by a correlated color temperature (CCT) of the light source, which is the temperature at which the heated black-body radiator matches the color or hue of the white light source. White light typically has a CCT of between about 4000 degrees Kelvin (K) and 8000K. White light with a CCT of 4000 has a yellowish color. White light with a CCT of 8000K is more bluish in color, and may be referred to as “cool white.” “Warm white” may be used to describe white light with a CCT of between about 2600K and 3700K, which is more reddish in color. “Neutral white” may refer to white light with a CCT of between about 3700K and 5000K.
The light from a single-color LED may be converted to white light by surrounding the LED with a wavelength conversion material, such as a phosphor. The term “phosphor” may be used herein to refer to any materials that absorb light in one wavelength range and re-emit light in a different wavelength range, regardless of the delay between absorption and re-emission and regardless of the wavelengths involved. A fraction of the light may also pass through the phosphor and/or be reemitted from the phosphor at essentially the same wavelength as the incident light, experiencing little or no wavelength conversion. In general, phosphors absorb light having shorter wavelengths and re-emit light having longer wavelengths. As such, some or all of the light emitted by the LED at a first wavelength may be absorbed by the phosphor particles, which may responsively emit light at a second wavelength. For example, a single blue-emitting LED may be surrounded with a yellow-emitting phosphor (such as cerium-doped yttrium aluminum garnet (YAG)), referred to herein as a blue shifted yellow (BSY) LED. The resulting light, which is a combination of blue light and yellow light, may appear white to an observer.
However, the light generated from a phosphor-based solid state lighting component including a blue-emitting LED and a yellow-emitting phosphor may have a relatively low CRI. As such, objects illuminated by the light from such a component may not appear to have natural coloring due to the limited spectrum of the light. While the CRI may be improved by including a red-emitting element, such as a red-emitting LED and/or a red-emitting phosphor, difficulties may arise in balancing the color point, CRI, and lumen output of a lighting module or apparatus including such lighting components.